This invention relates to an impeller such as a cross flow fan or sirocco fan of the type widely used as fans for air conditioners and other kinds of equipment.
Ordinarily, the construction of a cross flow fan, i.e., a kind of multiblade fan is like the one disclosed in Japanese Patent Unexamined Publication No. 60-17296 and Japanese Utility Model Unexamined Publication No. 59-167990.
That is, a cross flow fan has specific features unlike other types of fans whereby the direction in which air flows into the fan and the direction in which air flows out of the fan are generally reverse to each other, as indicated by the arrows in FIG. 1A, and the air flow rate can generally be increased in proportion to the axial length of the fan. For this reason, cross flow fans have recently come into wide use.
A technique of using a specific arrangement of impeller blades is known which resides in the fact that, as shown in FIG. 1B, impeller blades a are disposed at pitch angles determined on the basis of random numbers without any periodicity to reduce audible rotational noise (nz-sound, n: rotating speed, z: number of blades).
FIG. 2A shows an ordinary nz-sound frequency characteristic, and FIG. 8B shows an nz-sound frequency characteristic based on a random pitch arrangement of blades.
Japanese Utility Model Publication No. 60-12959 discloses the construction of a centrifugal fan.
FIG. 3 shows an example of this type of centrifugal fan which has a structure wherein fluid flows into the fan in the radial direction or obliquely at a fan inlet b and flows out in a spreading manner through an outlet c. This structure is suitable for use under high static pressures and high loads. A design in which each blade d is formed with an aerofoil section has also been adopted with a view to improving aerodynamic characteristics and flow rate characteristics.
Although the cross flow fan shown in FIG. 1 is designed to apparently reduce audible piping-like sounds by setting the pitch angles of the arrangement of the blades in an irregular manner so as to disperse the frequencies of nz-sounds as shown in FIG. 2B, n-sound (n: number of revolution) per one period becomes more sensible. This sound may increase the intensity of noise determined by the auditory sense, thereby impairing the noise reducing effects. Also, a problem of a reduction in the flow rate due to the irregularity of the blade pitch angles.
The centrifugal fan shown in FIG. 3 designed to improve efficiency by forming an aerofoil section of each blade may have a considerably large weight because the thickness of the blade is increased. If, on the other hand, a hollow blade structure is adopted, the number of manufacture steps is increased accordingly, resulting in an increase in the manufacture cost.